The present invention relates in general to alternators or alternator/starters of motor vehicles and, more particularly, to a device intended to produce a signal representative of the excitation current flowing through the rotor of such a rotary machine.
Prior Art
It is now conventional to equip a motor-vehicle heat engine with an electronic engine-monitoring device intended to continuously adjust the engine""s supply as a function of various measured conditions, and/or to [sic] an electronic device for supervising the activation and deactivation of electrical loads of the vehicle, which device is intended to keep the load on the alternator below a maximum level.
One of these conditions is the degree of excitation of the alternator or alternator/starter of the vehicle, which is itself representative of the braking torque applied by the alternator or alternator/starter to the heat engine. Hence, when such a measurement reflects a high resistive torque which is liable to stall the engine, the engine-monitoring device enriches the supply mixture of the engine in order to avoid maintaining [sic] a sufficient idling speed and to avoid the above-mentioned stalling. Alternatively, instruments which consume electricity from the on-board network of the vehicle may be temporarily disconnected.
Conventionally, the degree of excitation of the alternator is given by the profile of the excitation-control signal (typically a pulse-width modulation PWM signal) applied to the power semiconductor which, in combination with a freewheel diode, determines the current in the excitation winding.
There are hence solutions for providing the engine-monitoring device with this signal, either directly or in the form of an analogue or digital value representative of it. In particular, output signals conventionally referred to as xe2x80x9cFRxe2x80x9d for xe2x80x9cfield responsexe2x80x9d or xe2x80x9cDFxe2x80x9d for xe2x80x9cdigital fieldxe2x80x9d, which are intended for the engine-monitoring device, are known. Document FR-A-2 695 269 in the name of the Applicant Company describes, in particular, a device for generating an xe2x80x9cFRxe2x80x9d signal.
However, all the known devices which generate a signal based on the excitation-control signal are susceptible to the inherent variations in the effective excitation current as a function of temperature. More precisely, it is found that the ohmic resistance of the excitation winding varies very significantly as a function of temperature, and indeed a variation in this resistance by about 70% is typically observed when the temperature of the winding changes from 25xc2x0 C. (cold running) to 200xc2x0 C. (order of magnitude for hot running).
This variation in resistance greatly alters the actual excitation current Iexc for a given excitation-control signal. In a similar way, a particular current may correspond to very different profiles of the excitation-control signal, and therefore of the corresponding FR or DF signal, depending on whether a low temperature or a high temperature is involved.
Of course, an image of the effective current in the excitation winding could be obtained by employing a shunt in series with the winding, and by measuring the voltage across the terminals of this shunt. However, this would entail significant problems of, on the one hand, representativity of the measurement since the ohmic value of the shunt varies significantly with temperature, and, on the other hand, of heat dissipation which, in particular, prevents the shunt from being produced monolithically.
It is an object of the present invention to generate a signal representative of the current actually flowing through the excitation winding of an alternator, by using means which are suitable for being integrated monolithically, in particular with an excitation regulator, and in a way which is reliable and stable as a function of temperature.
To that end, the present invention provides, according to a first aspect, an alternator for a motor vehicle, comprising a circuit for regulating the current flowing through an excitation winding of the said alternator, the said regulator circuit comprising a switch which is controlled to be in the on mode or in the off mode and is in series with the excitation winding, and a freewheel component with one-way conduction which is connected to the terminals of the excitation winding, and a circuit for delivering, on an output, information representative of the state of excitation of the alternator, characterised in that the said circuit for delivering information representative of the state of excitation of the alternator comprises:
a means for generating, while the controlled switch is in the on state, a first signal proportional to the current flowing through the said controlled switch; and
a means for generating a second signal, this means being capable, while the controlled switch is in the on state, of generating the second signal on the basis of the first signal and, while the controlled switch is in the off state, of generating the second signal by storing the first signal which existed before the said controlled switch was turned off.
the current-measurement circuit includes means for keeping the three terminals (source, drain and gate [lacuna] of the excitation and measurement transistors at respectively identical potentials.
It will be noted here that the term xe2x80x9calternatorxe2x80x9d in the description above is intended to mean not only alternators per se, but also alternators which are capable of operating as a motor, such as alternator/starters.
Preferred, but non-limiting, aspects of the alternator according to the invention are as follows:
the means for generating the first signal comprises a current-mirror circuit comprising a first semiconductor switch, constituting the said controlled switch, and a second semiconductor switch.
the first and second semiconductor switches comprise first and second transistors made from identical elementary cells.
the second transistor comprises a number of elementary cells equal to a small fraction of the number of elementary cells of the first transistor.
the means for generating the first signal furthermore comprises a means for converting the current passing through the second semiconductor switch into a voltage, the said voltage constituting the first signal.
the means for generating the second signal comprises a means capable of generating digital information representative of the first signal while the controlled switch is in the on state, and a means for selectively storing the said digital value from the moment when the controlled switch becomes turned off.
the means for selectively storing the digital value comprises an incrementation/decrementation circuit which, on an input, receives a turn-off signal established on the basis of the current in the controlled switch becoming zero.
the means for generating the second signal furthermore comprises a comparator, which, on a first input, receives the first signal and, on a second input, receives an analogue signal obtained by conversion of the said digital information, and whose output is addressed to an input of the said incrementation/decrementation circuit intended to determine the direction of incrementation/decrementation.
the alternator furthermore comprises:
a means for generating, on an output terminal, a third signal which results from the said second signal and constitutes the said information representative of the state of excitation of the alternator.
the means for generating the third signal comprises a means for converting the second signal into an output current.
the means for generating the third signal also comprises a means for converting the output current into a reverse output current, and a selector capable of selectively operating the output-current conversion means.
the output-current conversion means comprises a current-mirror circuit.
the circuit for delivering information representative of the state of excitation of the alternator is produced monolithically.
the said circuit for delivering information representative of the state of excitation of the alternator is produced monolithically with the regulator circuit.
According to a second aspect, the present invention provides a regulator circuit intended to monitor the current flowing through an excitation winding of an alternator, the said regulator circuit comprising a switch which is controlled to be in the on mode or in the off mode and is intended to be connected in series with the excitation winding, and a freewheel component with one-way conduction which is intended to be connected to the terminals of the excitation winding, and a circuit for delivering, on an output, information representative of the state of excitation of the alternator, characterised in that the said circuit for delivering information representative of the state of excitation of the alternator comprises:
a means for generating, while the controlled switch is in the on state, a first signal proportional to the current flowing through the said controlled switch;
a means for generating a second signal, this means being capable, while the controlled switch is in the on state, of generating the second signal on the basis of the first signal and, while the controlled switch is in the off state, of generating the second signal by storing the first signal which existed before the said controlled switch was turned off.
Lastly, the invention provides, according to a third aspect, a circuit intended to deliver, on an output, information representative of the state of excitation of an alternator, the said circuit being intended to interact with a regulator circuit, which is itself intended to monitor the current flowing through an excitation winding of the said alternator and comprises a switch which is controlled to be in the on mode or in the off mode and is in series with the excitation winding, and a freewheel component with one-way conduction which is connected to the terminals of the excitation winding, characterised in that it comprises:
a means for generating, by coupling with the controlled switch and while the latter is in the on state, a first signal proportional to the current flowing through the controlled switch;
a means for generating a second signal, this means being sensitive to the on or off state of the controlled switch and being capable, while the controlled switch is in the on state, of generating the second signal on the basis of the first signal and, while the controlled switch is in the off state, of generating the second signal by storing the first signal which existed before the said controlled switch was turned off.